B介子衰變至三體重子之研究

Abstract

B介子三體重子衰變中最奇特的現象為重子對多以小角度、幾近平行地方式被產生出來。在重子對質譜中，這現象相對於重子對產生臨界點附近的一個最大衰變率。在本論文中我們提出以因式化假設來研究B介子三體重子衰變。根據重子對被產生的方式，因式化假設將此種衰變分成兩類：(1) 重子對由真空中被產生出來，及(2) 重子對由B介子轉變而來。對於第一種過程，因式化假設可以給出與實驗非常吻合的結果，包括正確地描述重子對產生臨界點附近的最大衰變率，及重子對的角度分布，顯示因式化假設可能為此類衰變的主要貢獻。對於其他包含第二類過程的衰變，最新的實驗結果顯示與前一類衰變迥異的重子對角度分布現象。由於這類過程的衰變振幅含有重子對及B介子的三體矩陣元素，此種矩陣元素被懷疑可能與上述奇異的角度分布相關，且可能是這類衰變的主要貢獻來源。由於目前並無處理此問題的恰當理論，我們只能由現象上得出對此種衰變過程的定性理解。我們還將因式化假設推廣到三體介子衰變問題上。在無可調參數的情況下我們得到與實驗幾乎一致的結果。因式化假設在此得到強有力的證實。最後我們也提出由B介子衰變實驗抽取出如介子形因子等物理量的構想。

We study three-body baryonic B decays. A unique feature about three-body baryonic B decays is the threshold enhancement, where a clear peak is seen near the threshold in the baryon pair mass spectra for the modes that have been observed so far. A generalized factorization approach has been proposed, where two mechanisms of the baryon pair production are given, with one involving baryon pairs produced by a current, and the other involving baryon pairs created by a current-induced transition from the B meson. Baryon form factors are needed in parameterizing the factorized matrix elements containing the baryon pairs. In the case of baryon pairs produced by currents, all the baryon form factors can be asymptotically related to the nucleon electromagnetic form factors by using SU(3) flavor symmetry and the asymptotic behavior of the form factors as predicted by perturbative QCD. With a simple parametrization for the nonperturbative part of the form factors, the results agree well with data, which suggests that factorization may be the dominant contribution in this kind of processes. For decays that also involve baryon pairs produced by the transitions from the B meson, one needs to deal with matrix elements that contain both the baryon pair and the B meson, for which we still do not have control. The angular distribution of B+ -> ppbarK+ reveals that the contribution from the transition part should be dominant in the factorization approach. The relevant data can be roughly described by a similar approach to the previous case. We also extend our approach to three-body B -> D(*)K-K(*)0 decays on account of the similarity between B0 -> D(*)+K-K0 and B0 -> D*-pnbar, one of the baryonic modes that factorization works well with. It is surprising to find that experiment already suggests factorization as the K-K(*)0 pair is found to be in the J^P = 1-+ state, which can be naturally accounted for as produced by the (axial)vector current. Furthermore, in the absence of tuning parameters, the calculated branching fractions of B 0 -> D(*)+K-K0 agree very well with data, signifying the evidence of factorization. A project of using the B0 -> D+K-K*0 data to extract the K-K*0 axial form factor is also carried out.